Chemistry Reference
In-Depth Information
Ta - M
α
80
70
60
50
40
30
20
10
0
Si-K
α
30
N K
C K
FWHM = 19.9 eV
25
20
Ta - M
β
15
10
O K
Ti-L
α
5
0
250
300
350
400
Energy (eV)
450
500
550
1550 1600 1650 1700 1750 1800 1850 1900 1950
Energy (eV)
Figure7.4.
Two parts of an X-ray spectrum taken with a TES microcalorimeter. A Ti-N spectrum
is shown left, and a Si-Ta spectrum is displayed right. All peaks are clearly separated by the TES
detector. A conventional Si(Li) or SDD showing the thin profile (right) is unable to resolve these
peaks. Figure from Ref. [53], reprinted with permission from the National Institute of Standards and
Technology.
8
μ
m. As a result of absorption, the cryogenic temperature is increased a
little and the magnetization of the paramagnetic sensor is decreased. The
planar sensor has a diameter of 50
μ
m and a height of 8
μ
m. Usually, a
solid solution of gold doped with about 500 ppm erbium (Au : Er) is
chosen. The magnetization of this alloy is caused by the magnetic moment
of erbium ions and is due to 4f electrons of the N-shell, which is only partly
filled and is well below the outer O- and P-shell of the ion. The 4f band
with only 11 instead of 14 electrons can be treated as a doublet with a gap
of 0.0015 eV.
The magnetization increases with the magnetic induction and decreases with
the temperature. At a low temperature of 50 mK and a small magnetic
induction of only 6 mT the alloy is magnetized with 150 A/m (equivalent
with an increase of the magnetic field strength). An incident photon with
energy of 6 keV raises the temperature by nearly 1 mK. Because of its low
impedance, the sensor is coupled with a SQUID. This detector also needs a
cryostat supplied with pumped liquid helium. The final temperature is reached
by means of an adiabatic demagnetization refrigerator with paramagnetic salt
pills. The complete device is protected against external magnetic fields by a
superconducting shield of lead. The energy resolution amounts to about 3 eV
for photons of 6 keV leading to a resolving power of 2000. Unfortunately, only a
low count rate of 2 cps can be treated so that a counting time of more than 1 h is
necessary in order to reach a measurement precision better than 1%.
Microcalorimeters can be used for building large arrays with small pixels,
which can serve as 2D detectors. Such devices have manifold applications for
X-ray astronomy in space when the temperature is near the absolute zero point
Search WWH ::
Custom Search